Advances in Material Research, Vol.2, No.3 (2013) 173-180

DOI: http://dx.doi.org/10.12989/amr.2013.2.3.173 173

Copyright © 2013 Techno-Press, Ltd.

http://www.techno-press.org/?journal=amr&subpage=7 ISSN: 2234-0912 (Print), 2234-179X (Online)

Structural, FTIR and ac conductivity studies

of NaMeO3 (Me ≡ Nb, Ta) ceramics

Sumit K. Roy1, S.N. Singh2, K. Kumar3 and K. Prasad4

1Department of Physics, St. Xavier’s College, Ranchi – 834 001, India

2University Department of Physics, Ranchi University, Ranchi – 834 008, India

3Refractory Division, RDCIS, SAIL, Ranchi – 834 002, India

4University Department of Physics, T. M. Bhagalpur University, Bhagalpur – 812 007, India

(Received May 19, 2013, Revised July 10, 2013, Accepted August 6, 2013)

Abstract. Lead-free complex perovskite ceramics NaMeO3 (Me ≡ Nb, Ta) were synthesized using conventional solid state reaction technique and characterized by structural, FTIR and electrical (dielectric and ac conductivity) studies. The crystal symmetry, space group and unit cell dimensions were determined from the experimental results using FullProf software. XRD analysis of the compound indicated the formation of single-phase orthorhombic structure with the space group Pmmm (47). Dielectric studies showed the diffuse phase transition at 394C for NaNbO3 and 430C for NaTaO3. Ac conductivity in both the compounds follows Jonscher‟s power law.

Keywords: NaNbO3; NaTaO3; perovskite; dielectric properties; lead-free; electroceramic

1. Introduction

Ceramics with ABO3-type of structure have been widely used in various electronic and

microelectronic devices such as capacitors, piezoelectric transducers, pyroelectric detectors/sensors,

memory devices, microwave tunable devices, electrorestrictive actuators, SAW substrates, MEMS,

etc. It has been observed that materials used for most of these applications are made from lead

bearing compounds such as lead titanate, lead zirconate titanate, lead magnesium niobates, etc. In

recent years, continuous efforts are being made worldwide to improve the performance

characteristics of lead-free ABO3-type materials suitable for practical applications (Seo et al. 2013,

Du et al. 2013a, Du et al. 2013b, Vendrell et al. 2013b, Bhagat et al. 2013, AmarNath and Prasad

2012, Kumari et al. 2011, Eichel and Kungl 2010, Damjanovic et al. 2010, Panda 2009, Rödel et

al. 2009, Wunderlich 2009, Shrout and Zhang 2007, Takenaka and Nagata 2005). Among them,

NaNbO3 and NaTaO3 are well-known pervoskite oxides and are considered to be feasible

alternatives to commercial Pb-containing piezoceramics (Koruza et al. 2010, Hsiao et al. 2007,

Chen et al. 2005, Reznitchenko et al. 2001, Kennedy et al. 1999). Further, the studies of dielectric

properties and electrical conductivity in such compounds are very important since the associated

physical properties are dependent on the nature and magnitude of conductivity in these materials.

Corresponding author, Ph.D., E-mail: k.prasad65@gmail.com; k_prasad65@yahoo.co.in

Sumit K. Roy, S.N. Singh, K. Kumar and K. Prasad

Accordingly, in the present work, the structural (X-ray and its Rietveld analysis), FTIR, dielectric

and ac conductivity properties of perovskite NaNbO3 and NaTaO3 (abbreviated hereafter NN and

NT, respectively) ceramics have been presented.

2. Materials and methods

The polycrystalline samples of NaMeO3 (Me ≡ Nb, Ta) were prepared by the conventional solid-state

reaction technique using the thermochemical reaction: )(235232 gCONaMeOOMeCONa . High

purity (>99.9%) carbonates/oxides of Na2CO3 (Merck, Germany) and Nb2O5 (Hi media, India) /

Ta2O5 (Aldrich, USA) were mixed in proper stoichiometric quantities. Wet mixing was carried out

with acetone as the medium for homogeneous mixing. Grinding was performed using a ball

milling apparatus for about 1 h. Well-mixed powders were then calcined at 850C and 1080C for

4 h, respectively for NN and NT. The as-calcined powders were compacted into thin (~1.5 mm)

cylindrical disks with an applied uniaxial pressure of 650 MPa. The pellets were then sintered in

air atmosphere at 900C for NN and at 1100

C for NT in alumina crucible for 2 h. The completion

of reaction and the formation of desired compounds were checked by X-ray diffraction technique.

The XRD data were obtained on sintered pellets of NN and NT with an X-ray diffractometer

(Rikagu Miniflex, Japan) at room temperature, using CuK radiation ( = 1.5405 Å ). The scanning

(2θ) was performed from 20-70 with a step of 0.04 at a scanning rate of 2.0/min. The 2θ vs.

intensity data were plotted with the WinPLOTR program and the angular positions of the peaks

were obtained with the same program (Roisnel and Rodrıguez-Carvajal 2000). The dimensions of

the unit cell, hkl values and space group of both the compounds were obtained using the DICVOL

program in the FullProf 2012 software and then refinements were carried out through the profile

matching routine of FullProf (Rodrıguez-Carvajal 2012). The Bragg peaks were modeled with

pseudo-Voigt function and the background was estimated by linear interpolation between selected

background points. The Fourier Transformed Infrared (FTIR) spectra of NN and NT samples along

with their constituent compounds were collected in the transmission mode using an Alpha-T

Bruker FTIR spectrophotometer in the range of 500-4500 cm-1

. The electrical measurements were

carried out on a symmetrical cell of type Ag|Ceramic|Ag, where Ag is a conductive paint coated on

either side of the pellets. Electrical impedance (Z), phase angle (), loss tangent (tanδ) and

capacitance (C) were measured as function of frequency (0.1 kHz–1 MHz) at different

temperatures (35C–500C) using a computer-controlled LCR Hi-Tester (HIOKI 3532-50, Japan)

interfaced with a microprocessor controlled dry temperature controller (DPI-1100, Sartech Intl.,

India) at a heating rate of 4C/min. AC conductivity data were thus obtained using the relation:

tan2 oac f where f, , and tan are operating frequency, dielectric constant and loss

tangent respectively.

3. Results and discussion

X-ray diffraction patterns of NN and NT along with their Rietveld refinement profiles are

shown in Fig. 1. It can be seen that the observed and calculated XRD profiles for both the

compounds are in fair agreement, suggesting the formation of single phase compounds having

orthorhombic structure with space group Pmmm(47). The crystal data and refinement factors of

174

Structural, FTIR and ac conductivity studies of NaMeO3 (Me ≡ Nb, Ta) ceramics

Table 1 The crystal data and refinement factors of NaNbO3 and NaTaO3 obtained from X-ray powder

diffraction data

Parameters NaNbO3 NaTaO3

Crystal System

Space group(No.)

a (Å )

b (Å )

c (Å )

V (Å3)

Rp

Rwp

Rexp

RB

RF

χ2

d

QD

S

Orthorhombic

Pmmm(47)

5.5659

5.5071

3.8792

118.9057

25.6

22.3

10.5

0.219

0.133

4.499

1.2964

1.8378

2.123

Orthorhombic

Pmmm(47)

5.5262

5.4812

3.8961

118.0126

34.6

33.1

9.66

0.358

0.451

11.7

1.6524

1.8392

3.427

Description of parameters

Rp (profile factor) = 100[Σ|yi-yic|/Σ|yi|], where yi is the observed intensity and yic is the calculated intensity at

the ith

step.

Rwp (weighted profile factor) = 100[Σωi|yi-yic|2/Σωi(yi)

2]

1/2, where 2/1 ii and 2

i is variance of the

observation.

Rexp (expected weighted profile factor) = 100[(n-p)/Σωi(yi)2]

1/2, where n and p are the number of profile

points and refined parameters, respectively.

RB (Bragg factor) = 100[Σ|Iobs-Icalc|/Σ|Iobs|], where Iobs is the observed integrated intensity and Icalc is the

calculated integrated intensity.

RF (crystallographic RF factor) = 100[Σ|Fobs-Fcalc|/Σ|Fobs|], where F is the structure factor, F = √(I/L), where L

is Lorentz polarization factor.

χ2 = Σωi(yi-yic)

2.

d (Durbin–Watson statistics) = Σ{[ωi(yi-yic)-ωi-1(yi-1-yic-1)]2}/Σ[ωi(yi-yic)]

2.

QD = expected d.

S (goodness of fit) = (Rwp/Rexp).

both NN and NT, obtained from XRD data, are depicted in Table 1. This result is in consistent with

the similar system (Taub et al. 2013, Sindhu et al. 2012, Zhao et al. 2012). A little difference in the

unit cell parameters as well as unit cell volume of NN and NT were observed, which may be due

to the difference in the ionic radii of Nb5+

(0.78 Å ) and Ta5+

(0.64 Å ) ions.

Fig. 2 shows the FTIR spectra of the perovskites (a) NaNbO3 and (b) NaTaO3 along with the constituent carbonate (Na2CO3) and oxides (Nb2O5/Ta2O5). The IR spectra of the Na2CO3

comprises of a weak band at 686 cm-1

which corresponds to in plane 2

3CO bending, a relatively strong band at 866 cm

-1 which corresponds to out of plane 2

3CO bending along with a very weak band and an intense peak at 1068 cm

-1 and at 1409 cm

-1 which corresponds to C-O symmetric

stretching. The bands at 1639 cm-1

and 3443 cm-1

are assigned to the bending of O-H-O and O-H stretching modes of vibration due to physically adsorbed water molecules on the surface of the sample during palletisation with KBr. Further, in case of Nb2O5 two broad bands at 620 cm

-1 and

175

Sumit K. Roy, S.N. Singh, K. Kumar and K. Prasad

828 cm-1

are observed which are assigned to stretching modes of Nb-O and Nb=O vibrations (Rojac et al. 2012, Mathai et al. 2012). Also, the FTIR spectrum of Ta2O5 the metal oxide (Ta-O)

bonds ranges in the lower frequency region of 600 cm-1

to 760 cm-1

can easily be seen. With the exception of slight variation in the position, the peaks of the starting carbonates and oxides are consistent with the vibrations characteristic of 2

3CO , Me-O and Me=O and are in agreement with the literature data (Rojac et al. 2012). Furthermore, the peaks within the range 500 cm-

1 to 900 cm

-1 of

the starting oxides and carbonates are not present in the product compounds and new prominent peaks at 633 cm

-1 (in NaNbO3) and 557 cm

-1 and 672 cm

-1 (in NaTaO3) can be observed which

corresponds to the Nb-O and Ta-O bonds thereby confirming the formation of desired compound (Rojac et al. 2012). Both the samples under investigation contain the multiple peaks (A-O absorption bands) in the frequency region 1000-1640 cm

-1, which are due to the presence of Na-O

vibrations. Further, IR spectra are very sensitive to the presence of moisture during the course of experimentation. A broad and strong peak can easily be seen around 3400-3500 cm

-1 which are due

to free water molecules (H2O bands) and strong stretching (antisymmetric and symmetric) modes

of the OH-group (Jha and Prasad 2012, Kumar et al. 2013). The temperature dependence of dielectric constant (ε) and loss tangent (tanδ) at different

frequencies for NN and NT are shown in Fig. 3. The plots for both the compounds show diffuse

-2000

0

2000

4000

6000

8000

20 30 40 50 60 70-4000

-2000

0

2000

4000

6000

8000

10000

YObs

YCal

YObs

-YCal

Bragg position

Inte

ns

ity

(a

rb.

un

it)

NaTaO3

NaNbO

3

Bragg angle (deg.)

Fig. 1 Rietveld refined patterns of NaNbO3 and NaTaO3. Symbols represent the observed data

points and the solid lines their Rietveld fit

500 1000 1500 2000 2500 3000 3500 4000 4500

0.30.40.50.60.70.80.91.01.1

0.5

0.6

0.7

0.8

0.9

1.0

1.1

0.30.40.50.60.70.80.91.01.1

Wavenumber ( cm-1 )

NaNbO3

(a)

1200 1400 1600 18000.70

0.75

0.80

0.85

0.90

0.95

1.00

Tra

ns

mit

tan

ce

Wavenumber ( cm-1 )

Na2CO

3

Tra

nsm

itta

nce

Nb2O

5

500 1000 1500 2000 2500 3000 3500 4000 4500

0.30.40.50.60.70.80.91.01.1

0.5

0.6

0.7

0.8

0.9

1.0

1.1

0.8

0.9

1.0

Wavenumber ( cm-1 )

NaTaO3

(b)

Na2CO

3

1200 1400 1600 18000.65

0.70

0.75

0.80

0.85

0.90

0.95

1.00

Tra

ns

mit

tan

ce

Wavenumber ( cm-1 )

Tra

nsm

itta

nce

Ta2O

5

Fig. 2 FTIR spectra for (a) NaNbO3 and (b) NaTaO3 ceramics along with their constituent

compounds

176

Structural, FTIR and ac conductivity studies of NaMeO3 (Me ≡ Nb, Ta) ceramics

phase transition (DPT) Tm, respectively, at 394C (εm = 578) and 430C (εm = 395). Similar results

have been found by Koruza et al. (2010). Also, the maximum value of ε decreases and -T curves

flatten with the increase in frequency. The broadening in the dielectric peak is a common

occurrence in such type of materials. The multiple ion occupation at different sites, microstructure

and sintering process causes deviation from normal Curie-Weiss behaviour, where Tm is not sharp,

but physical properties change rather gradually over a temperature range, and is known as DPT.

The DPT can be explained by modified Curie-Weiss law: γ

CT-TA /ε/ε )(11 max , where γ is called

the diffusivity parameter giving the measure of broadness in phase transition and normally varies

between 1 and 2 (Prasad et al. 1996). The values of γ are estimated to be and 1.27 and 1.89,

respectively for NN and NT using the method of least squares, which confirms DPT. The values of

and tan at room temperature are, respectively, found to be 105 and 0.093 for NN and 211 and

0.21 and NT at 1 kHz.

Fig. 4 shows the log-log plots of ac electrical conductivity versus frequency at different temperatures. The ac conductivity spectrum for both NN and NT shows dispersion throughout the chosen frequency range and with the increment in temperature plots get flattened (plateau value). The switch from the frequency-independent to the dependent regions shows the onset of the

conductivity relaxation phenomenon which indicates the translation from long range hopping to the short range ion-motion (Mizaras et al. 1997). This observation can be well expressed by the Jonscher‟s universal power law: ζac = ζdc + Aω

s , where ζdc is the frequency independent (dc) part

of conductivity (Jonscher 1983). The second term in the right hand side is the frequency sensitive region with 0 ≤ s ≤ 1. It was found that the value of index s decreases with the rise of temperature and is always less than 1. The model based on hopping of charge carriers

)}/1ln(/{61 oBMB TkWTks , where, WM is the energy required to cross the barrier height, which predicts a decrease in the value of the index „s‟ with the increase in temperature and so found to be consistent with the experimental results. Therefore, the conduction in both the systems could be considered due to the short-range translational type hopping of charge carriers (AmarNath and Prasad 2012).

It is observed that the hopping conduction mechanism is generally consistent with the

0 100 200 300 400 5000

50

100

150

200

250

300

350

400

450

500

550

600

650

NaNbO3

1 kHz

10 kHz

100 kHz

1 MHz

1 kHz

10 kHz

100 kHz

1 MHz

Temperature (OC)

Die

lec

tric

co

ns

tan

t

0

2

4

6

8

10

Ta

ng

en

t lo

ss

0 100 200 300 400 5000

50

100

150

200

250

300

350

400

450

NaTaO3

1 kHz

10 kHz

100 kHz

1 MHz

1 kHz

10 kHz

100 kHz

1 MHz

Temperature (OC)

Die

lec

tric

co

ns

tan

t

0

2

4

6

8

10

Ta

ng

en

t lo

ss

Fig. 3 Temperature dependence of dielectric constant and tangent loss of NaNbO3 and NaTaO3

at different frequencies

177

Sumit K. Roy, S.N. Singh, K. Kumar and K. Prasad

0.1 1 10 100 100010

-7

10-6

10-5

10-4

10-3

500OC

100OC

NaNbO3

Frequency ( kHz )

ac (

S/m

)

0.1 1 10 100 100010

-7

10-6

10-5

10-4

10-3

10-2

500OC

ac (

S/m

)

Frequency ( kHz )

NaTaO3

100OC

Fig. 4 Frequency dependence of ac conductivity of NaNbO3 and NaTaO3 at different

temperatures

1.2 1.3 1.4 1.5 1.6 1.7 1.8 1.9 2.0 2.1-14

-13

-12

-11

-10

-9

-8

-7

Experimental (NaNbO3)

Experimental (NaTaO3)

Linear fit

ln

ac

1000/T (K-1)

Fig. 5 Temperature dependence of ac conductivity of NaNbO3 and NaTaO3 at 1 kHz

existence of a high density of states in the materials having band gap like that of semiconductor. Due to localization of charge carriers, formation of polarons takes place and the hopping conduction may occur between the nearest neighboring sites. Fig. 5 shows the variation of ac conductivity versus 10

3/T for NN and NT. The nature of variation is almost linear indicating the ohmic

nature of contact and conductivity obeys the Arrhenius relationship: )/exp( TkE Baoac , where Ea is the apparent activation energy of conduction, kB is the Boltzmann constant and T is the absolute temperature. The nature of variation shows the negative temperature coefficient of resistance (NTCR) behaviour in NN and NT. The values of Ea for NN and NT are estimated, respectively, to be 0.40 eV and 0.53 eV at 1 kHz using a linear least square fitting. The low value of Ea may be due to the carrier transport through hopping between localized states in a disordered

manner (Prasad et al. 2006, AmarNath and Prasad 2012).

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Structural, FTIR and ac conductivity studies of NaMeO3 (Me ≡ Nb, Ta) ceramics

4. Conclusions

Polycrystalline samples of NaNbO3 and NaTaO3, prepared through a high temperature solid

state reaction route, were found to possess perovskite type orthorhombic structure with the space

group Pmmm(47). Dielectric studies showed the diffuse phase transition at 394C for NN and

430C for NT. It is observed that ac conductivity exhibits Jonscher‟s power law and the carrier

transport follows hopping type conduction mechanism in both the compounds.

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